Stereoscopic image projection system

ABSTRACT

A liquid crystal display projection system for pictures that can be viewed stereoscopically, comprising: a source of a repetitive three color sequence of light; first and second imagers responsive to respective drive signals representative of the same picture from different angles of view; at least a first polarizing beam splitter for separating said repetitive three-color sequence of light into a P polarized three color sequence of light and an S polarized three color sequence of light, said S polarized three color sequence of light illuminating said first imager and said P polarized three color sequence of light illuminating said second imager; a first filter for removing P polarized light directed to said first imager; and, a second filter for removing P polarized light reflected from said second imager, wherein said light reflected from said first imager and said light filtered by said second filter form a stereoscopic image when viewed through respective P and S polarizing lenses.

This application claims the benefit, under 35 U.S.C.§365 ofInternational Application PCT/US03/02244, filed Jan. 27, 2003, which waspublished in accordance with PCT Article 21(2) on Aug. 7, 2003 inEnglish and which claims the benefit of European patent application No.02001865.1, filed Jan. 28, 2002.

This invention relates to the field of light engines for projectionsystems, and more particularly, to a light engine architecture thatenables stereoscopic viewing.

The existing technology in projection systems is inefficient, requiringmajor optical systems to transform the light into a usable form. Arclamps and other similar light sources are by their nature broadband inoutput and therefore generate infrared, ultraviolet, and non-primaryvisible light, as well as the red, green, and blue light which is usefulfor projection. The inefficiencies of color filters used to process thislight also leads to broader band colors and therefore smaller colorspace. Light sources such as arc lamps also produce random “mixed”polarization, and therefore require additional optical system componentsto handle polarization separation. To further enhance “étendue”, acomplex system of integrators and collimators are required to transforma focused beam from a light source (such as an arc lamp) into a uniformrectangular illumination. Étendue is generally known as the product ofradiant flux density and the area of a radiating or receiving surface.This is used to determine absolute values for the emitted (reflected ortransmitted) energy, in order to control the overall energy balance. Inaddition, since light coming from the lamps is essentially white,dedicated dichroic filters would be necessary to produce red, green, andblue light necessary for a projection system. As a result of all thehardware required to overcome the problems described above, a large,bulky optical system would be needed for the purpose of achievingadequate through-put of light through a typical light engine. Even withall the existing schemes to increase the throughput of light through alight engine, the best systems achieve between 40–60% throughput. Manyexisting systems use a single LCOS panel for each color, and attempt tomaximize the illumination with the appropriate polarization usingpolarization recovery systems, such as PBS arrays and other expensive orinefficient schemes. Thus, a need exists for a light engine whichsubstantially increases system through-put in terms of light whileadding minimal cost.

Other systems utilize more than one imager or panel for each color,requiring at least four imagers. Three of the imagers must be alignedwith the fourth imager. Each of the three imagers must be aligned withrespect to six degrees of movement. This is extraordinarily difficult,not only because the pixel size is on the order of only 10 microns, andeach respective pixel of each imager must be aligned perfectly to enablesharp pictures, but each imager is subject to thermal stress andmovement as the light engine heats up. Thus, a further need exists for alight engine that not only can substantially increase system throughput, but is much less complex and much less expensive to build, alignand operate.

It should also be noted that existing stereoscopic displays typicallyrequire special glasses for viewing the stereo image. Several techniqueshave been around including using Red and green to separate left andright eye images of a monochrome picture. This should work if you have ared/green viewer. Another technique is frame sequential left and righteye images with glasses that incorporate LCD shutters that switch insynchronism with the image. The glasses are active and so require powerand control circuitry, and a timing signal from the display. Anothertechnique involves polarization to separate the left and right eyeimages. This is the technique used in the cinema. Its use withelectronic displays is less prevalent but can be implemented using anelectrically switchable polarizer in front of the display and framesequential images. The implementation would be comparable to the LCDwhere a filter would be used to obtain color in frame sequential CRTdisplays. The latter two approaches require frame sequential left andright images. It is essential that no residual image from one eyeremains on the display when the image is switched to the other. Thuswith present technology they can only be implemented with fast CRTdisplays. Thus, a further need exists for a method of viewing astereoscopic image using polarization that would not require specializedlenses.

In a first aspect of the present invention, a device for generating astereoscopic image using a liquid crystal display projection systemcomprises at least a first imager and a second imager and at least afirst polarizing beam splitter for substantially separating P polarizedlight from S polarized light and directing the P polarized light to thefirst imager and the S polarized light to the second imager. The systemfurther comprises a first filter for filtering out a substantial portionof any P polarized light reflected from the first imager and a secondfilter for filtering out a substantial portion of any P polarized lightin the S polarized light split out by the at least first polarizing beamsplitter.

In a second aspect of the present invention, a light engine arrangementfor generating a stereoscopic image comprises at least a first imagerand a second imager and a means of substantially separating P polarizedlight from S polarized light and directing the P polarized light to thefirst imager and the S polarized light to the second imager. The lightengine arrangement further comprises a first filter for filtering out asubstantial portion of any P polarized light reflected from the firstimager and a second filter for filtering out a substantial portion ofany P polarized light in the S polarized light split out by the means ofsubstantially separating.

In a third aspect of the present invention, a method of viewing astereoscopic image using polarization comprises the steps of driving afirst imager with a first image signal modulated for S polarization anddriving a second imager with a second image signal modulated for Ppolarization, wherein the first image signal and the second image signalcombined provide a stereoscopic view for projection by a projectionlens. The method further comprises the steps of injecting unpolarizedlight into the system and separating P polarized light from S polarizedlight from the injected unpolarized light. Additionally, the methodfurther comprises the steps of directing the P polarized light to thefirst imager and directing S polarized light reflected from the firstimager toward the projection lens after filtering the reflected Spolarized light for stray P polarized light and directing the Spolarized light toward the second imager after filtering the S polarizedlight for stray P polarized light and directing reflected P polarizedlight reflected from the second imager toward the projection lens.

FIG. 1 is a block diagram of a light engine architecture in accordancewith the present invention.

FIG. 2 is another block diagram of a light engine architecture usingWire Grid Polarizer in accordance with the present invention.

FIG. 3 is a flow chart illustrating a method of increasing brightness ina light engine and optionally being able to view images stereoscopicallyin accordance with the present invention.

FIG. 4 is a block diagram of the light engine architecture of FIG. 1displaying information corresponding to a right eye image with a firstimager in accordance with the present invention.

FIG. 5 is a block diagram of the light engine architecture of FIG. 1displaying information corresponding to a left eye image with a secondimager in accordance with the present invention.

Referring to FIG. 1, a novel architecture 10 for generating astereoscopic image using polarization with a light engine is shown. Thearchitecture disclosed further significantly increases system throughputin terms of light brightness. While existing systems attempt to increasethroughput using a single panel or imager, the present inventionutilizes at least a second panel or imager. Adding a second panel to thesystem, and using a color wheel as is done with Digital MicromirrorDevice (DMD)-type systems effectively doubles the total luminous flux.Unpolarized light is injected into the system. P polarized light isseparated from S polarized light at the first PBS, wherein P polarizedlight goes to one imager and S polarized light goes to another one.Hence there is a polarization recovery operated by the addition of oneimager or panel. The cost penalty is that the additional panel isneeded, and a method of achieving proper alignment of the two separateimagers is required. The cost of an additional imager is well offset bythe overall savings in cost. The two panels can be driven with the samesignal to enhance system throughput, for example doubling the lightoutput. Alternatively, the two imagers can be driven by two differentsignals to obtain the stereoscopic effect previously mentioned.

Such a system achieves the advantages of reducing complexity and costbecause the invention can be embodied with only two imagers, and canprovide almost as much light through put as systems having four or moreimagers.

A liquid crystal display projection system, architecture or system 10comprises at least a first imager 22 and a second imager 20. Preferably,these imagers are liquid crystal on silicon (LCOS) display devices. Thesystem 10 can further comprise at least a first polarizing beam splitter14 for substantially separating P polarized light from S polarized lightand directing the P polarized light to the first imager 22 and the Spolarized light to the second imager 20. Additionally, the systemcomprises a first filter 26 for filtering out a substantial portion ofany P polarized light reflected from the first imager 22 and a secondfilter 24 for filtering out a substantial portion of any P polarizedlight in the S polarized light split out by the at least firstpolarizing beam splitter 14. Preferably, the system includes four PBSsincluding the first PBS 14, a second PBS 18 for directing S polarizedlight split from the first PBS toward the second imager, a third PBS 16for directing S polarized light reflected from the first imager towardsa fourth PBS 20, wherein the fourth PBS 20 directs S polarized lightreflected from the first imager and P polarized light from the secondimager towards a projection lens 28. In such a system, the liquidcrystal display projection system 10 would further preferably include afirst quarter wave plate 23 between the third PBS 16 and the firstimager 22 and a second quarter wave plate 21 between the second PBS 18and the second imager 21. It should be understood that the use ofquarter wave plate could be obviated in a system using a wire gridpolarizer as will be further discussed below. The system furthercomprises a source of unpolarized light 12 such as an arc lamp androtatable color wheel 13 placed between at least the first PBS and thesource of unpolarized light. The lamp and color wheel are a means forsupplying a repetitive sequence of colored light, for example red,green, blue, red, green, blue and so on. The rotating wheel andcorresponding drive signals supplied to the imagers are synchronized asis known in the art.

One basic advantage of the system of the present invention is that evenif the étendue is increased by the polarization splitting, it does notend up in a loss of brightness as a second imager is used to increasethe system's étendue and match it to that of the illumination whensignals S1 and S2 are the same drive signal. Alternatively, signals S1and S2 can be different in order to produce a polarization based stereovision experience. In the stereoscopic embodiment, the user needs onlypolarizing glasses, not LCD shutters. This dual panel system wouldrequire added mechanical complexity and alignment for the second paneland the corresponding drivers for the panels.

Referring again to FIG. 1, the first filter 26 and second filter 24advantageously and substantially increase image contrast, a significantproblem in the prior art because there can not be an analyzer at theoutput just before the lens or a clean up polarizer at the input. Thefirst and second filters serve as clean up polarizers in each channel.The role of these clean up polarizers are discussed further on anumerical example in the alternative embodiment discussed belowregarding wire grid PBSs instead of glass PBS. It should be understoodwithin contemplation of the present invention that additional filterscould be used. For example, additional filters (not shown) between theinterface of PBS 14 and PBS 16 and between the interface of PBS 18 andPBS 20 could even further improve contrast as well and clean up theillumination from further unwanted polarization. These filters could bedye type or wire grid polarizers. It should also be noted that filtersor polarizers set in the light reflected off the imagers act asanalyzers that are used to clean up residual polarization from theimagers (see filter 26 or a filter (not shown) located between PBS 18and PBS 20). Since imagers are imperfect and could reflect backelliptical polarized light instead of linear light, the additionalfilters described herein would be suitable when the PBSs in the imagingpath of the light do not clean enough of the residual polarization.

Referring to FIG. 2, the light engine arrangement or system 50 can bemade out of wire grid polarizers (WGPs) made by the company Moxtek. AWGP is constructed by forming a series of closely-spaced aluminum barson a glass substrate. In other words, a WGP is constructed of a thinlayer of aluminum wires on a glass substrate that makes for anexceptional PBS. As with PBSs, this grid reflects light of onepolarization state while transmitting light of the orthogonalpolarization state. This reflective property coupled with the WGP'srugged construction enables many new uses in projection systems. WGPsare less expensive than glass PBSs. The WGP as shown can be constructedin sheet format or sandwiched between other components. For a wire gridPBS, the typical transmission of P-polarized light is of 85% (bluechannel, incidence angle of 45 degrees), so that 15% of P-polarizedlight is reflected off the PBS. The system 50 includes the wire gridpolarizer 52 and a first imager 56 in a direct path for the illuminationwhere arrow 51 represents the unpolarized light provided as an inputinto system 50. A second imager 58 receives illumination on anall-reflected path. For the direct path, 0.85²=0.7225 of the P polarizedlight from the input illumination reaches the first imager 56. In ablack state, the first imager reflects ideally all that light, and onthe output, it bounces twice on the WGP 52 before going out of thesystem. Hence, the black state light throughput is 0.85²×0.15². For awhite state, the first imager 56 is rotating the polarization to S, andthen as the typical reflection coefficient for S is of 0.0033, the whitestate light throughput is 0.85²×0.99672, so that the contrast is finally0.9967²/0.152=44:1, where contrast is determined by dividing the whitestate light throughput by the black state light throughput.Unfortunately, contrast ratio is extremely low. A similar calculationgives exactly the same result for the other channel. The maincontributor to the low contrast is the P reflected light on both wiregrid PBSs represented by the dashed arrows.

The present invention enhances the contrast by canceling the tworeflected P polarization states in both channels by adding clean uppolarizers or filters 54. Dye polarizers could be used if the irradianceon their surface does not destroy them, but preferably wire gridpolarizers are used such as the Proflux brand WGP. Preferably, a hightransmission WGP can be oriented so that it just transmits 0.10% of Plight and could be placed after the first reflection of P polarizedlight in both channels. In this case, the black level falls down to0.85²×0.15²×0.001, and the white state to 0.85²×0.9967²×0.835, where0.835 is the typical transmission of the high transmission wire gridpolarizer for the white state polarization. The contrast in this case isabove 35000:1. Thus, without the clean up polarizers or filters 54, lossof contrast will result due to a high residual reflection of P-polarizedlight in the black state. With the clean up polarizers, the contrast canbe boosted by a factor of 500–1000. The use of WGPs in the embodiment ofFIG. 2 also obviates the need to use quarter wave plates as was used inthe embodiment of FIG. 1 where conventional glass PBSs were used. Itshould also be noted that the grids having wires 53 and 55 on the WGPs52 and 54 respectively should preferably be oriented as shown in FIG. 2.

Referring to FIG. 3, a flow chart illustrating a method 100 ofincreasing brightness in a liquid crystal display projection system isshown. At step 102, unpolarized light is injected into the system.Optionally, at step 103, the first imager and the second imager could bedriven with different signals to produce polarization based stereovision. As another option at step 104, the unpolarized light could besubjected to a rotating color wheel before injecting the unpolarizedlight into the system. Then, P polarized light is separated from Spolarized light from the injected unpolarized light at step 105. At step106, the P polarized light is directed to a first imager and reflected Spolarized light reflected from the first imager is directed toward aprojection lens after filtering the reflected S polarized light forstray P polarized light. As previously shown in FIG. 2, the dashedarrows illustrate the stray P polarized light. At step 108, the Spolarized light is directed toward a second imager after filtering the Spolarized light for stray P polarized light and directing reflected Ppolarized light reflected from the second imager toward the projectionlens. At this point, light reflected from the first and second imagerscan be combined at step 110. If the images were modulated appropriately,the first and second images can be combined to provide a stereoscopicimage using polarization.

Referring to FIGS. 4 and 5, the first imager 22 displays informationcorresponding to a first image intended for a right eye of a viewer. Thesecond imager 20 displays information corresponding to a second imageintended for a left eye of a viewer. This arrangement results in Spolarized light directed to the right eye and P polarized light directedto the left eye. To filter any unwanted P polarized light that would goto the right eye of the viewer or any unwanted S polarized light goingto the left eye of the viewer, the viewer would wear polarized glasses.The polarized glasses would have a polarizer 70 oriented so that only Spolarized light would reach the right eye and another polarizer 80oriented so that only P polarized light would reach the left eye. Ofcourse, it should be understood within contemplation of the presentinvention that stereoscopic images could equally be generated and viewedby reversing the corresponding polarizations and images as needed.

It should be understood that the present invention could described in amyriad of different other arrangements within the scope of the claims orthat other imagers could be used other than LCOS microdisplays asdescribed herein. Although the present invention has been described inconjunction with the embodiments disclosed herein, it should beunderstood that the foregoing description is intended to illustrate andnot limit the scope of the invention as defined by the claims.

1. A liquid crystal display projection system for pictures that can beviewed stereoscopically, comprising: means for supplying a repetitivethree color sequence of light; first and second imagers responsive torespective drive signals representative of the same picture fromdifferent angles of view; at least a first polarizing beam splitter forseparating said repetitive three-color sequence of light into a Ppolarized three color sequence of light and an S polarized three colorsequence of light, said S polarized three color sequence of lightilluminating said first imager and said P polarized three color sequenceof light illuminating said second imager; a first filter for removing Ppolarized light directed to said first imager; and, a second filter forremoving P polarized light reflected from said second imager, whereinsaid light reflected from said first imager and said light filtered bysaid second filter form a stereoscopic image when viewed throughrespective P and S polarizing lenses; wherein the at least firstpolarizing beam splitter comprises four polarizing beam splittersincluding the first polarizing beam splitter, a second polarizing beamsplitter for directing S polarized light split from the first polarizingbeam splitter toward the second imager, a third polarizing beam splitterfor directing S polarized light reflected from the first imager towardsa fourth polarizing beam splitter, wherein the fourth polarizing beamsplitter directs S polarized light reflected from the first imager and Ppolarized light from the second imager towards the projection lens; andwherein the system further comprises a first quarter wave plate betweenthe third polarizing beam splitter and the first imager and a secondquarter wave plate between the second polarizing beam splitter and thesecond imager.
 2. The system of claim 1, wherein the first imager andthe second imager are liquid crystal on silicon display devices.
 3. Thesystem of claim 1, comprising only said first and second imagers.
 4. Thesystem of claim 3, wherein the system further comprises a color wheelplaced between at least the first polarizing beam splitter and thesource of unpolarized light.
 5. The system of claim 1, wherein thesystem further comprises a projection lens.
 6. The system of claim 5,wherein the at least first polarizing beam splitter comprises a wiregrid polarizer.
 7. The system of claim 1, wherein the liquid crystaldisplay projection system further comprises a pair of polarizing glassesworn by a viewer of the device wherein the polarizing glasses has aright lens polarizer oriented to allow only S polarized light throughand a left lens polarizer oriented to allow only P polarized lightthrough.
 8. The system of claim 7, wherein the S polarized light passingthrough the right lens polarizer corresponds to the right image and theP polarized light passing through the left lens polarizer corresponds tothe left image.